The invention relates to radio receivers having the capability of receiving digital television (DTV) signals transmitted using quadrature amplitude modulation (QAM) of the principal carrier wave.
A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Subcommittee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television Subcommittee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*106 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, xe2x88x92S, xe2x88x92S and +S. The value +S is one level below the maximum positive data excursion, and the value xe2x88x92S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511 -sample pseudo-random noise sequence (or xe2x80x9cPN511 sequencexe2x80x9d) followed by three 63-sample pseudo-random noise sequences (or xe2x80x9cPN63 sequencesxe2x80x9d). The middle PN63 sequence in the training signal is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one""s complementary respective to each other. The other PN63 sequences and the PN511 sequence are transmitted in accordance with the first logic convention in the first line of each data field, whether odd-numbered or even-numbered. The initial line of each data field includes other information, such as a code for indicating the mode of the VSB transmission, and such as an indication of trellis code information at the close of the previous data field.
The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded after having been randomized and subjected to diagonal byte interleaving. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a ⅔ rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of xe2x88x927, xe2x88x925, xe2x88x923, xe2x88x921, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of xe2x88x92S is xe2x88x925.
VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than using VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving QAM transmissions of DTV as well as receiving VSB transmissions of DTV.
Radio receivers for receiving digital television signals, in which receiver the final intermediate-frequency signal is somewhere in the 1-8 MHz frequency range rather than at baseband, are described by C. B. Patel et alii in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER, and included herein by reference. The use of infinite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,548,617 issued Aug. 20, 1996, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The use of finite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,731,848 issued Mar. 24, 1998, entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING NG FILTERS, AS FOR USE IN AN HDTV RECEIVER, and incorporated herein by reference. The design of receivers for both VSB and QAM signals in which both types of signal are processed through the same intermediate-frequency amplifiers receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 to C. B. Patel et alii and entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER is incorporated herein by reference. U.S. Pat. No. 5,659,372 issued Aug. 19, 1997 to C. B. Patel et alii and entitled DIGITAL TV DETECTOR RESPONDING TO FINAL-IF SIGNAL WITH VESTIGIAL SIDEBAND BELOW FULL SIDEBAND IN FREQUENCY is incorporated herein by reference. U.S. patent application Ser. No. 08/266,753 filed Jun. 28, 1994 by C. B. Patel et alii and entitled RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS is incorporated herein by reference. U.S. patent application serial U.S. Pat. No. 5,748,226 issued May 5, 1998 to A. L. R. Limberg and entitled DIGITAL TELEVISION RECEIVER WITH ADAPTIVE FILTER CIRCUITRY FOR SUPPRESSING NTSC CO-CHANNEL INTERFERENCE is incorporated herein by reference. These patents and patent applications are all assigned to Samsung Electronics Co., Ltd., pursuant to employee invention agreements already in force at the time the inventions disclosed in these patents and patent applications were made.
In the radio receivers described in U.S. Pat. No. 5,506,636 the final intermediate-frequency signal is digitized, and synchrodyne procedures to obtain baseband samples are carried out in the digital regime. In radio receivers that are to have the capability of receiving digital TV signals no matter whether they are transmitted using VSB or QAM, frequencies of the local oscillators in the tuner can remain the same no matter whether VSB or QAM transmissions are being received. U.S. Pat. No. 5,506,636 indicates that this is possible because the differences in carrier frequency location within the channel can be accommodated in the synchrodyning procedures carried out in the digital regime. The radio receivers described in U.S. Pat. No. 5,506,636 and claimed in the claims following this specification do not synchronously detect the in-phase QAM sidebands and the quadrature-phase QAM sidebands in the analog regime and then digitize the two sets of baseband results in respective analog-to-digital conversion circuits for application to digital equalization filtering. Rather, to avoid the problems with accurately tracking two analog-to-digital conversion circuits at high sampling rates, a single analog-to-digital converter digitizes the final intermediate-frequency signal and the resulting stream of digital samples is converted to complex digital samples including a stream of real digital samples and a stream of imaginary digital samples. These complex digital samples and complex digital samples descriptive of the suppressed carrier are then multiplied in a complex digital multiplier to generate real and imaginary baseband results that are already in digital form circuits for application to digital equalization filtering.
The invention is embodied in apparatus for reproducing data from a selected digital television signal transmitted in packet form as quadrature-amplitude-modulation (QAM) of a carrier. This apparatus has a tuner for selecting said digital television signal and converting it to a final intermediate-frequency signal, which is digitized by an analog-to-digital converter before synchrodyning to baseband in the digital regime. QAM synchrodyning circuitry responsive to the digitized final intermediate-frequency signal generates real and imaginary digital sample streams of QAM symbol code, by synchrodyning the final IF signal to baseband in the digital regime. Symbol decoding circuitry responsive to the real and imaginary digital sample streams of QAM symbol coding generates a digital data stream. Data synchronization recovery circuitry recovers data synchronizing information included in the digital data stream. A video decoder of decompression type responsive to portions of the digital data stream, as selected in response to the data synchronizing information, generates digital signals descriptive of red, green and blue video signals. The video decoder can be an MPEG-2 decoder, for example, or one for decoding video compressed using wavelet descriptions rather than direct-cosine-transform descriptions, by way of further example.